This invention relates to reciprocating-piston internal combustion engines of the type which employs a catalyst in the or each combustion chamber to initiate and promote combustion at the compression temperature in the chamber. Such engines are referred to herein as catalytic combustion engines.
One object of the present invention is to provide an internal combustion engine capable of operation at a compression ratio higher than is normal practice for a spark-ignited gasoline engine using high-octane-number commercial gasoline, but very much lower than the minumum required for compression-ignition of the injected fuel in a Diesel engine.
For example, it can be shown that the optimum compression ratio of an i.c. engine for brake power output is around 12:1. This is based on the fact that the ideal thermal efficiency of an i.c. engine follows a law of the type ##EQU1## where R is the compression ratio and .gamma. is the ratio of the specific heat of air at constant pressure to that at constant volume. The efficiency .eta. of the engine when plotted against compression ratio R shows an initially-rapid rise which progressively flattens off as the compression ratio increases. The consequence of increasing the compression ratio is to increase the motoring loss of the engine, i.e. the ideal indicated mean effective pressure will have a greater mean pressure element due to friction subtracted from it to arrive at the net or brake mean effective pressure which represents the actual output. The optimum value of compression ratio having regard to the brake power output is found to be around 12:1.
This has been demonstrated practically with spark-ignited gasoline engines, but with compression ratios of around 12:1 these require the use of special gasolines having a high resistance to "pre-ignition", i.e. uncontrolled self-ignition prior to the instant of spark ignition of the charge, and to "knock" due to self-ignition of part of the charge in a remote part of the combustion chamber after the normal ignition of the charge by the timed spark. Such fuels are not widely available commercially. The use of compression ratios of this order in production gasoline engines is therefore not viable.
The normal Diesel engine requires a very high compression ratio to raise the temperature in the combustion chamber to a value at the end of compression which will permit the self-ignition of the diesel fuel spray when injected into the combustion chamber shortly before top-dead-centre. As the swept volume of the individual cylinders is reduced a higher compression ratio becomes necessary to offset the loss of charge heat during compression. For example in a small compression-ignition engine having a Ricardo Comet Mark V precombustion chamber (British Pat. No. 786,329) a compression ratio of around 21:1 is necessary to give satisfactory cold starting (using a heater plug) and high-speed light load operation without misfiring, using diesel distillate as the injected fuel. It would be quite impossible to operate such an engine with a compression ratio as low as 12:1.
It would moreover be quite impossible to operate such a compression-ignition engine on gasoline of 91 RON (Research Octane Number) or on methanol (methyl alcohol) with RON&gt;100 as the injected fuel, at a compression ratio of 12:1 or even as high as 21:1. This is because good quality gasoline such as is normally used in a spark-ignited engine has a high self-ignition temperature which is the chief characteristic of its high octane number, and in consequence has either a long ignition delay period or will fail to ignite altogether when injected into an engine cylinder under conditions of compression pressure and temperature such that diesel fuel would readily ignite.